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United States Patent |
5,242,551
|
Frank
,   et al.
|
September 7, 1993
|
Electron induced transformation of an isoimide to an n-imide and uses
thereof
Abstract
A method of isomerizing an isoimide to an n-imide is described wherein an
electron is supplied to the isoimide which induces the isomerization and
wherein the isomerization is catalytic to the electron which remains
available to initiate further isomerization. A polyimide is deposited onto
a conductive substrate by providing a composition containing a
polyisoimide and an electrolyte providing the substrate and a counter
electrode in the composition, and providing a bias between the substrate
and counter electrode to thereby supply an electron to the polyisoimide
which isomerizes to deposit the insoluble polyimide on the substrate.
Inventors:
|
Frank; Ernest R. (Madison, WI);
O'Toole; Terrence R. (Hopewell Junction, NY);
Viehbeck; Alfred (Stormville, NY)
|
Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
Appl. No.:
|
771285 |
Filed:
|
October 3, 1991 |
Current U.S. Class: |
548/417; 204/157.81; 205/419; 257/E23.119; 548/418; 548/423; 548/433 |
Intern'l Class: |
C25B 003/04 |
Field of Search: |
204/59 R,181.4,181.6,181.7,157.15,157.81
549/520
548/417,418,423,433
|
References Cited
U.S. Patent Documents
H729 | Feb., 1990 | Wallace et al. | 528/183.
|
2980694 | Apr., 1961 | Sauers et al. | 549/320.
|
3041376 | Jun., 1962 | Sauers et al. | 549/320.
|
3821170 | Jun., 1974 | Harnson | 204/72.
|
3940322 | Feb., 1976 | Phillips et al. | 204/59.
|
4132715 | Jan., 1979 | Roth | 548/549.
|
4171302 | Oct., 1979 | Abblard et al. | 548/549.
|
4331705 | May., 1982 | Samudrala | 427/54.
|
4369247 | Jan., 1983 | Goff et al. | 430/311.
|
4551522 | Nov., 1985 | Fryd et al. | 204/159.
|
4568601 | Feb., 1986 | Araps et al. | 427/43.
|
4654223 | Mar., 1987 | Araps et al. | 427/385.
|
4656050 | Apr., 1987 | Araps et al. | 427/385.
|
4699803 | Oct., 1987 | Araps et al. | 427/385.
|
4741988 | May., 1988 | Van der Zande et al. | 204/299.
|
4749621 | Jun., 1988 | Araps et al. | 428/437.
|
4871619 | Oct., 1989 | Araps et al. | 525/420.
|
4871619 | Oct., 1989 | Araps et al. | 427/12.
|
5021129 | Jun., 1991 | Arbach et al. | 205/126.
|
Foreign Patent Documents |
0318840 | Nov., 1988 | EP.
| |
Primary Examiner: Niebling; John
Assistant Examiner: Mayekar; Kishor
Attorney, Agent or Firm: Morris; Daniel P.
Parent Case Text
DESCRIPTION
This application is a continuation-in-part of application Ser. No.
07/676,660, filed Mar. 28, 1991.
Claims
What is claimed is:
1. A method comprising:
providing a molecule containing an isoimide group conjugated to an aromatic
moiety; and providing electron density to said isoimide group which
initiates an isomerization of said isoimide group to an n-imide group
conjugated to an aromatic moiety.
2. The method of claim 1, wherein said electron density is
electrochemically provided.
3. The method of claim 1, wherein said electron density is chemically
provided.
4. The method of claim 1, wherein said molecule is in a liquid containing a
member selected from the group consisting of an electrolyte, a reducing
agent and mixtures thereof and wherein said reducing agent has an
oxidation potential that is negative with respect to the reduction
potential of said molecule.
5. The method of claim 4, further including, providing an electrically
conductive substrate in said liquid, providing a counter electrode in said
liquid and providing an electrical bias between said substrate and said
counter electrode to thereby deposit an isomer of said molecule having an
n-imide group conjugated to an aromatic moiety onto said electrically
conductive substrate.
6. The method of claim 5, wherein said molecule is a polyisoimide molecule.
7. The method of claim 6, wherein said polyisoimide is formed from
pyromellitic dianhydride and oxydianiline.
8. The method of claim 6, wherein said said polyisoimide is formed from
benzophenone tetracarboxylic dianhydride and
1,3-bis(2-aminophenoxy)benzene.
9. The method of claim 4, wherein said reducing agent is selected from the
group consisting of anthracene anion, N-butylphthalimide anion, benzil
anion, benzophenone anion, benzoin dianion, and sodium naphthalenide,
anion of N,N'-di-n-butyl-pyromellitimide.
10. The method of claim 4, wherein said electrolyte contains a cation
selected from the group consisting of tetraalkylammonium,
tetraalkylphosphonium, alkali metal, aryl-alkyl-ammonium,
tetraalkylammonium and chelated metal.
11. The method of claim 10, wherein said electrolyte contains an anion
selected from the group consisting of tetrafluoroborate,
hexafluorophosphate, aryl sulfonate, perchlorate and halide.
12. The method of claim 4, wherein said liquid contains an aprotic solvent
or solvent mixture.
13. The method of claim 12, wherein said aprotic solvent or mixture is
selected from the group consisting of nitrile compound, nitro compound,
amide, cyclic amide, ester, cyclic ester, ether, oxide and sulfo compound.
14. The method of claim 12, wherein said aprotic solvent or mixture is
selected from the group consisting of N,N dimethylformamide,
N-methyl-2-pyrrolidone and tetrahydrofuran.
15. The method of claim 4, wherein said concentration of said polyisoimide
in said liquid is about 0.0005 M to about 5 M.
16. The method of claim 4, wherein the concentration of the electrolyte in
said liquid is about 1 to about 0.01 M.
17. The method of claim 4, wherein the concentration of the electrolyte in
said liquid is about 0.2 to about 0.05 M.
18. The method of claim 4, wherein the concentration of the reducing agent
is about 0.001 M to about 0.05 M.
19. The method of claim 4, wherein said substrate is a metal selected from
the group consisting of palladium, platinum, silver, gold, copper, cobalt
and nickel.
20. The method of claim 4, wherein said substrate is copper.
21. The method of claim 4, wherein said substrate is a conductive polymer,
conductive glass, or superconductor or semiconductor.
22. The method of claim 4, wherein said bias generates an initial working
electrode potential that is about 50 mV or more negative than the
reduction potential of said polyisoimide, followed by a bias of between 50
mV to 2 V positive of the polyimide reduction potential.
23. The method of claim 4, wherein said counter electrode is platinum.
24. The method of claim 4, wherein said electrolyte is tetrabutylammonium
tetrafluoroborate.
25. The method of claim 4, wherein said reducing agent is benzophenone
anion.
26. The method of claim 1, wherein said electron density is photochemically
provided.
27. The method of claim 1 wherein said electron density is provided by
providing an electron to said isoimide group.
28. The method of claim 1, wherein said electron density transforms said
isoimide group into a reduced isoimide group having an electron which
isomerizes to a reduced n-imide group having an electron which is
transferred to another isoimide group to form an n-imide group and another
reduced isoimide group.
Description
FIELD OF THE INVENTION
The present invention is concerned with transforming a molecule containing
an isoimide group to a molecule containing an n-imide group and uses
thereof. More particularly, the present invention is directed to
depositing a polymeric material and especially a polyimide onto a
substrate in its imidized form, more particularly, onto a conductive
substrate. The present invention is particularly concerned with
electrodeposition of a polyimide onto a substrate from a solution
containing polyisoimide.
BACKGROUND
Polyimides have widespread industrial use. In particular, polyimides are
commonly employed in the semiconductor and packaging industry. Polyimides
are usually employed for metal passivation or conductor insulation,
particularly, because the polyimides exhibit low dielectric
characteristics along with high thermal and chemical stability.
In packaging of semiconductor chips, polyimide films are often coated onto
substrates. Typically, a polyamic acid or alkyl ester precursor of the
polyimide is applied by spin coating onto the desired substrate, and
subsequently cured by thermal excursions of up to about 400.degree. C.
However, one problem associated with such a process is that the polyimide
precursors employed are reactive with metals such as copper. This in turn
causes oxidation of the metal which leads to the incorporation of metal
oxide into the polymer bulk during the curing cycle. The presence of the
metal oxide adversely affects the dielectric properties of the polyimide
and the reliability of the metal-polyimide interface. Another problem
associated with applying the polyamic acid or polyamic ester precursor is
that the heating results in imidization, i.e., ring closure with
concurrent release of water (for polyamic acids) or alcohol (for polyamic
esters).
This curing process results in weight loss and dimensional changes in the
polymer such as shrinking. This concern can be minimized by applying a
preimidized polyimide coating. However, most polyimides, especially those
possessing the best packaging properties, are not soluble and therefore
cannot be applied in the imidized form.
A polyimide has an imide group which has a variety of isomers. We have
discovered that by supplying an electron to an isoimide to form a reduced
isoimide it is energetically disposed toward transformation to an n-imide.
It is believed that the originally supplied electron remains on the
n-imide as a reduced imide which permits electrochemical deposition of the
reduced n-imide onto an electrically conducting substrate.
We have additionally discovered that the n-imide can use the initially
supplied electron for initiating the isomerization of another isoimide
molecule to an n-imide molecule. Therefore, the originally supplied
electron is not lost but remains in the system. The isoimide
transformation to the n-imide is catalytic to the supplied electron which
remains in the system.
A polyisoimide is generally more soluble in solvents than is poly-n-imide
which is often insoluble. Therefore, starting with a solution of
polyisoimide which, when reduced, is energetically disposed to isomerize
to a poly-n-imide which readily deposits into an electrically biased
conductor, is a substantially more efficient process than starting with a
solution of reduced poly-n-imide. The enhanced efficiency comes from the
enhanced isoimide solubility and the catalytic isoimide to n-imide
transformation which re-supplies the electron for use in additional
transformations. In this way, more rapid deposition can be achieved. The
original electron is preferably supplied from an electrode in solution to
form a reduced isoimide when starting with an isoimide and to form a
reduced imide when starting with an n-imide. In the former situation the
electron is available for re-use in another transformation where as in the
latter situation the electron is not available for reducing another
poly-n-imide.
It is an object of this invention to transform an isoimide to an n-imide by
supplying an electron to an isoimide which is then energetically disposed
toward transformation to the n-imide.
It is another object of this invention to electrolytically deposit an imide
compound onto a conducting surface starting with an isoimide.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention is a method of transforming
an isoimide molecule to an n-imide molecule by providing electron density
to the isoimide molecule which is thereby energetically disposed to
transforming into an n-imide molecule.
In a more particular aspect of the present invention, the electron is
provided to the isoimide molecule by providing an electron thereto.
In another more particular aspect of the present invention, the electron
density is provided to the isoimide molecule by providing an agent capable
of providing electron density overlap with the isoimide molecular
orbitals.
In a more particular aspect of the present invention, the isoimide molecule
is provided in a solution containing a biased electrode from which an
electron is transferred to the isoimide which is energetically disposed to
transforming the n-imide molecule.
In another more particular aspect of the present invention, the isoimide
molecule is provided in a solution containing a chemical or
electrochemically generated reducing agent which transfers an electron to
the isoimide molecule which is energetically disposed to transforming to
the n-imide molecule.
In another more particular aspect of the present invention, polyimides in
n-imidized forms can be deposited onto a surface, in particular, onto a
conductive surface. The present invention makes it possible to deposit
polyimides onto conductive surfaces, such as copper circuitry, without
experiencing the problems discussed above associated with employing
polyimide precursor polyamic acid material.
The present invention makes it possible to deposit a cured polyimide onto
an electrically conductive substrate.
In particular, the present invention comprises depositing a coating onto an
electrically conductive substrate by providing a liquid composition
containing an electrolyte and/or reducing agent and a polyisoimide
providing an electrically conductive substrate in the liquid composition;
and providing an electrical bias between the substrate and the counter
electrode to thereby deposit the polyimide onto the electrically
conductive substrate. Patterned deposition can be achieved using a
conductive surface which contains a non-conductive patterned layer such as
an imaged photoresist thereby exposing only certain regions of the
conductive surface. These and other objects features and advantages will
become more apparent from the following detailed description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows polyimide thickness versus reduction time for deposition from
a solution of isoimide.
DETAILED DESCRIPTION
Molecules useful to practice the present invention contain an imide group
conjugated to an aromatic moiety having the structures of equations 1 and
2. Equation 1 is the isoimide isomer and equation 2 is the n-imide isomer.
##STR1##
The polyisoimides employed pursuant to the present invention are capable of
being reduced, i.e., accepting electrons. Normally, the polyisoimides
undergo electron transfer processes at relatively moderate potential (e.g.
-0.7 to -1.3 V vs. saturated calomel electrode, sce). Polyisoimides are
often substantially more soluble than poly-n-imide isomers. In their
reduced form, polyimides are usually much more soluble than in their
neutral, oxidized form.
Polyisomides can be generated from poly-n-imides by methods described in
copending U.S. application Ser. No. 07/770,576 filed on Oct. 3, 1991 the
teaching of which is incorporated herein by reference. The structure of
the isoimide isomer is readily apparent by reference to equation 1 and 2.
Briefly, this application describes a process for improving the reactivity
of a polyamic acid with a nucleophile by reacting the polyamic acid with
an isoimidizing agent to form a polyisoimide and then reacting the
polyisoimde with the nucleophile. In one embodiment, polyamic acid is
obtained by hydrolyzing a polyimide to the polyamic acid. The various
nucleophiles that can be reacted with the polyisoimide obtained comprise
organic hydroxy compounds or organic amines. In another embodiment, an
amic acid or a polyamic acid such as a polyamic acid obtained by the
hydrolysis of the surface of a polyimide is isoimidized with an acylhalide
of a heterocyclic nitrogen compound or an acyl halide of a heterocyclic
sulfur compound. This isoimidization reaction may be carried out in the
presence of a heterocyclic nitrogen compound which can act as a solvent
for the reactants. The isoimides, including the polyisoimides obtained,
are also reacted with nucleophiles such as organic hydroxy compounds or
organic amines.
The polyimides that can be employed in accordance with the present
invention include unmodified polymides, as well as modified polyimides
such as polyester imides, polyamide-imide-esters, polyamide-imides,
polysiloxane-imides, as well as other mixed polyimides. Such are
well-known in the prior art and need not be described in any great detail.
Generally, the polyimides include the following recurring unit:
##STR2##
where n is an integer representing the number of repeating units to
provide a molecular weight usually about 10,000 to about 100,000. R is at
least one tetravalent organic radical selected from the group consisting
of:
##STR3##
R.sup.2 being selected from the group consisting of a divalent aliphatic
hydrocarbon radicals having from 1 to 4 carbon atoms and carbonyl, oxy,
sulfo, sulfide, ether, siloxane, phosphine oxide, hexafluoroisopropylidene
and sulfonyl radicals and in which R.sup.1 is at least one divalent
radical selected from the group consisting of an aliphatic organic radical
or from the group shown:
##STR4##
in which R.sup.3 is a divalent organic radical selected from the group
consisting of R.sup.2, silico, and amino radicals. Polymers containing two
or more of the R and/or R.sup.1 radicals, especially multiple series of
R.sup.1 containing amido radicals can be used.
Polyimides are available commercially from a variety of suppliers including
(a) fully cured pre-imidized polyimide films (e.g., DuPont Kapton.RTM.
film); (b) fully cured preimidized powders (e.g., Ciba-Geigy Matrimid
5218.RTM.); and (c) polyimide precursors, most notably polyamic acids
(e.g. DuPont 2545 and 2611) and polyamic esters. The chemistry of
commercial polyimides examples of many of the components listed above, but
preferred polymers for use pursuant to the present invention are based on
the monomers pyromellitic dianhydride (PMDA) and oxydianiline (ODA, also
named 4,4'-diaminodiphenyl ether) or 3,3'4,4'-benzophenone tetracarboxylic
dianhydride (BTDA) and diamino-1,3,3-trimethyl-1-phenylindan (DAPI). Other
polymers for use pursuant to the present invention are the polymers of
3,3'-biphenylenetetracarboxylic acid (BPDA) and 1,4-phenylenediamine
(PDA). Polyimide films based on PMDA-ODA are available from Allied
Corporation under the tradename Apical.RTM. and from DuPont under the
tradename Kapton.RTM.. Polyimides based on BTDA-DAPI are available from
Ciba Geigy as XU-218.RTM.. Films based on BPDA-PDA are available from Ube
Corporation as Upilex.RTM. and from Hitachi Chemical Company as
PIQ-L100.RTM. Other tradename polyimides useful pursuant to the present
invention include Durimid.RTM. from Rogers Corporation.
All of the above examples of polyimides or polyimide precursors can be
readily converted to polyisoimides by methods known in the art.
Acetylene terminated polyisoimides are commercially available from National
Starch as the Thermid IP.RTM. series which is based on the monomer BTDA
and 1,3-bis(2-aminophenoxy)benzene (APB).
Although applicants do not want to be limited to a particular theory,
applicants believe that the self catalytic isomerization from isoimide to
n-imide proceeds according to one of the following two sequences of
equations where ISO is isoimide, n-Im is n-imide, e is an electron and
ISO.sub.a and ISO.sub.b refer to two different ISO molecules:
SEQUENCE I
ISO+e.fwdarw.ISO.sup.- Eq. 3
ISO.sup.- .fwdarw.n-Im.sup.- Eq. 4
n-IM.sup.- +ISO.fwdarw.n-Im+ISO.sup.- Eq. 5
SEQUENCE II
ISO.sub.a +e.fwdarw.ISO.sub.a.sup.- Eq. 6
ISO.sub.a.sup.- +ISO.sub.b .fwdarw.ISO.sub.a.sup.- +n-Im Eq. 7
According to Sequence I in equation 3, an electron is supplied to an
isoimide molecule to form a reduced isoimide molecule, ISO.sup.-, which,
according to equation 4 isomerizes with a certain rate constant, k, which
is expected to be on the order 0.1 M.sup.-1 .sup.-1 or faster, to a
reduced n-imide, n-IM.sup.-. According to the equation 5, the reduced
n-imide molecule interacts with another ISO molecule to reduce it by
transfer of an electron to generate another reduced isoimide which again
will isomerize to an n-imide according to equations 4 and 5. Therefore,
the originally supplied electron is reused in equations 4 and 5 to
initiate more than one transformation of an isoimide to an n-imide.
The added electron density of the reduced isoimide gives it enhanced
nucleophilic character. The general structure of a reduced isoimide group
conjugated to an aromatic moiety is shown in equation 8.
##STR5##
According to Sequence II, it is this enhanced nucleophilic character of the
reduced isoimide which initiates isomerization. In equation 6, an electron
is supplied to isoimide molecule, ISO.sub.a, to become reduced
ISO.sub.a.sup.- which in equation 7 interacts with isoimide molecule
ISO.sub.b to initiate isomerization of ISO.sub.b to an n-imide molecule
n-IM. ISO.sub.a.sup.- remains available to initiate another
isomerization.
According to the present invention, a solution containing the polyisoimide
and an electrolyte and/or reducing agent is employed. The function of the
optional reducing agent is to facilitate electron transfer from the
electrode to the polymer by acting as a mediator. When a reducing agent is
employed, such must have an oxidation potential that is negative with
respect to the reduction potential of the polyisoimide. Compounds such as
benzil anion, anthraquinone anion, benzophenone anion, benzoin dianion,
sodium naphthalenide, anion of N,N'-di-n-butylpyromellitimide,
tetrakis(dimethylamino) ethylene and even solvated electrons can be used
as the reducing agent.
The reducing agents can be generated in situ by electrochemical means.
Alternatively, catalytically activated reducing agents such as those
commonly employed in electroless metallization (e.g. formaldehyde,
borohydides . . .) can be used.
Examples of suitable organic compounds that can be electrochemically
reduced to provide the chemical reducing agent include, but are not
limited to, the following groups of compounds: unsaturated aromatic
hydrocarbons (e.g., anthracene), aldehydes and ketones (e.g., benzaldehye,
dibenzoylmethane), imides (e.g., N-n-butylphthalimide,
N,N'-di-n-butyl-3,3',4,4'-biphenyl tetracarboxylic diimide), carbodiimides
(e.g., bis-(p-chlorophenyl carbodiimide), aromatic heterocyclic nitrogen
compounds (e.g., 9.10-diazaphenanthrene), anhydrides (e.g., 1,8-naphthalic
anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride), quinones
(e.g., 9,10-anthraquinone) quaternary aromatic nitrogen compounds (e.g.,
N-p-biphenylbenzalimine), immonium salts (e.g., N-ethyl-N-methyl
benzophenone immonium salt) azo compounds (e.g., 4,4'-azobiphenyl), amine
oxides (e.g., acridine N-oxide), and organometallic compounds (e.g.,
dibiphenylchromiun (I) iodide).
Benzil, N-butylphthalimide, benzophenone and anthracene are examples of
specific compound that can be reduced to provide the chemical reducing
agents suitable for carrying out the present invention. The compounds can
be reduced by applying such to an electrochemical cell containing an anode
and a cathode and then applying a voltage.
Typically, electrochemical reduction is done using a two-compartment cell
whereby the compartments are separated by a sintered glass disk frit
having a porosity of less than 8 .mu.m. A salt bridge or semi-permeable
membrane also could be used to separate the compartments. The working
compartment is housed with a cathode electrode which is comprised of a
metal such as platinum, mercury, or stainless steel. The anode electrode
is comprised of a conductor such as platinum, carbon, or stainless steel.
For potentiostatic operation, an appropriate reference electrode is
positioned in the working compartment (e.g., a saturated calomel electrode
(SCE)). The cell can be purged with an inert gas such as N.sub.2 or argon
using an inlet tube and one-way valve or operation can be done in a glove
box under an inert atmosphere.
Electrochemical reduction of the polyisoimide or generation of the reducing
agent is accomplished by either galvanostatic, potentiostatic, or
voltage-controlled electrolysis. Typically, the current density range for
galvanostateic reduction is 0.1 to 2 mA/cm.sup.2. In potentiostatic mode,
reduction is typically done by applying a potential to the cathode which
is more negative (e.g., -50 mV or more) than the reduction potential for
the organic compounds as measured against the same reference electrode.
In addition to the polyisoimide and optional reducing agent, the solution
will include an electrolyte and preferably an electrolyte salt that
contains as cation a member from one of the following groups:
tetraalkylammonium, tetraalkylphosphonium, alkali metal,
aryl-alkylammonium, or chelated metal. The preferred tetraalkylammonium
group is tetrabutylammonium, but other tetraalkyls with alkyl group being
methyl, ethyl, propyl, isopropyl, pentyl, hexyl or mixed alkyl thereof can
be employed if desired. An example of a typical aryl group is phenyl and
an aryl-alkylammonium is benzyltributylammonium. An example of a chelated
metal cation is potassium 18-crown-6. The electrolyte salts preferably
contains as an anion one of the following:tetrafluoroborate,
hexafluorophosphate, aryl sulfonate, perchlorate, or halide such as
bromide or iodide. The solution containing the polyimide is preferably
comprised of an aprotic solvent or solvent mixture. The requirements for
the solvent are that it:
1. solvate electrolyte sufficiently to carry out electrochemical processes;
2. solvate the polyisoimide in its neutral and reduced forms;
3. not solvate the poly-n-imide in its neutral, oxidized form;
4. not react chemically with the polyisoimide or polyimide in either its
reduced or neutral form.
The aprotic solvents suitable for use in this invention may include, but
are not limited to, the following: nitrile and nitro compounds (e.g.,
acetonitrile, benzonitrile, nitromethane), amide and cyclic amide
compounds (e.g., N,N-dimethylformamide, N-methylformamide,
N,N-diethylformamide, N-ethylformamide, N,N-dimethylacetamide,
N-methyl-2-pyrrolidone, hexamethylphosphoramide), ester, cyclic ester, and
ether compounds (e.g., tetrahydrofuran, propylene carbonate, ethylene
carbonate, gammabutyrolactone, ethyl acetate tetrahydrofuran,
dimethylether), oxide and sulfo compounds (e.g., dimethylsulfoxide,
acetone, sulfolane, dimethylsulfone).
Although reference is made to a solution employing the polyimide, and
reducing agent and/or electrolyte, it is understood that separate
solutions of such can be admixed to provide the desired composition.
The concentration of the polyisoimide in the solution is usually about 1 to
30% by weight.
The concentration electrolyte in the solution is usually about 1 to about
0.01 M, preferably about 0.2 to about 0.05 M.
When employed, the concentration of the reducing agent is usually about
0.001 M to about 0.05 M and preferably about 0.002 M to about 0.005 M.
The substrate that is to be coated with the polyimide is placed in the
solution containing the corresponding isoimide isomer and is an
electrically conductive material, such as palladium, platinum, silver,
gold, copper, cobalt and nickel, with copper being preferred. The present
invention can also be employed to deposit polyimide onto conductive
polymers such as polyaniline, polypyrrole, and polythiophene, onto
conducting glass, and onto superconductors and semiconductors.
By following the process of the present invention it is possible to
selectively coat only predetermined areas of a substrate whereby only
exposed conductive material is coated and those portions of the substrate
that do not include conductive material and/or have the conductive
material already masked with a material or resin which could not be
activated by the process of the present invention will not be coated. For
example, photoresist material which can be used for patterned deposition
include Waycoat SC (Hunt Chemical) and KTFR (Kodak). This is of importance
for the fabrication of advanced second level circuit cards which typically
contain through vias that are metallized permitting electrical connection
between top and bottom surfaces. High density cards also often contain
buried metal layers or metal cores which must be electrically isolated
from the metallized vias. In fabricating the vias by drilling, punching,
etching, or ablating, subsequent insulation of the exposed metal core is
often problematic. By direct electrodeposition of imidized polyimide,
these exposed metal regions can be coated to isolate the core. This
approach also permits the application of the same polyimide used for the
top and bottom insulating layers.
When a bias is applied, the conductive substrate will function as an
electrode whereat the polyimide will deposit with it giving up electrons
and reverting back to its neutral state. Also provided in the solution is
a counter electrode with a bias applied between it and the substrate. The
counter electrode is a material that is relatively non-reactive in the
system such as platinum or carbon.
The first electron-reduction potential E.degree. for various polyimindes
are given:
______________________________________
E.degree..sub.1
______________________________________
PMDA-ODA (Kapton .RTM. )
-0.78
BPDA-PDA (Upilex .RTM. )
-1.34
BTDA-DAPI.sup.1 (XU-218 .RTM. )
-1.04
BTDA-APB.sup.3 (Thermid .RTM. )
-0.96
NTDA-ODA.sup.2 -0.64
______________________________________
E.degree. is referenced versus the saturated calomel electrode and
determined by cyclic voltammetry in 0.1 M tetrabutylammonium
tetrafluoroborate in acetonitrile.
The corresponding reduction potentials for the polyisoimides are about
0-100 my more positive (easier to reduce) than the polyimides.
1. BTDA-DAPI is 3,3',4,4'-benzophenone tetracarboxylic
dianhydride-diamino-1,3,3-trimethyl-1-phenylindan which is commercially
available from Ciba-Geigy under the tradename XU-218.RTM.
2. NTDA-ODA is 1,4,5,8-naphthalene tetracarboxylic
dianydride-4,4'-oxydianiline.
3. BTDA-APB is 3,3',4,4'-benzophenone tetracarboxylic
dianhydride-1,3-bis-(2-aminophenoxy) benzene which is commercially
available from National Starch and Chemical Company under the tradename
Thermid.RTM..
The process is usually carried out at room temperature under inert
atmosphere for about 1 to 15 minutes depending upon the desired thickness
of polyimide.
Alternatively it is contemplated that an electron or electron density can
be provided to the isoimide group by an electrogenerated nucleophile. The
electrogenerated nucleophile may simply be the reduced form of an organic
or inorganic molecule which does not have the reducing power for complete
electron transfer, but can donate a significant portion of its new found
electron density. Alternatively, the nucleophile may be generated by
decomposition of a molecule following electron transfer. For example,
reduction of a Pt(II) complex containing amine ligand will result in the
deposition of Pt(O) and freeing the amines.
Alternatively it is contemplated that an electron or electron density can
be photochemically provided to an isoimide group by photoreducing agents
or photogenerated nucleophiles. Examples of known photoreducing agents
include certain bipyridyl complexes of ruthenium, osmium and rhenium, and
also aromatic amines. Examples of photogenerated nucleophiles include
Co(III) complexes of amines where photolysis labilizes the amine ligand.
The method of providing an electron to an isoimide group described herein
above are exemplarly only and not limiting.
The following non-limiting examples are presented to further illustrate the
present invention:
EXAMPLE 1
Demonstration of Isoimide Isomerization Initiated by Direct Electron
Transfer
Thin films (2000-5000 .ANG.) of BPDA-PDA and PMDA-ODA polyamic acids were
spin applied to optically transparent electrode and isoimidized by
exposure to trifluoroacetic anhydride. Following isoimidization, the
polymers were a very bright yellow with .lambda. max .apprxeq.390 nm.
Electrochemical reduction through either the first or second polymer
reductions in an acetonitrilis solution containing 0.1 M
tetrabutylammonium tetrafluoroborate (TBABF.sub.4) resulted in loss of the
characteristic yellow color. Infrared analysis of the resulting films (KBR
pellet) showed no isoimide and only n-imide stretches.
EXAMPLE 2
Demonstration of isoimide isomerization mediated by a chemical reducing
agent
Thin films (2000-5000 .ANG.) of BPDA-PDA and PMDA-ODA polyamic acid were
spin-applied to CaF.sub.z substrates and isoimidized. The polyisoimide
films were chemically reduced by TBABF.sub.4 immersion into an
acetonitrite solution containing 0.1 M and 0.002 M benzophenone anion
(E(1/-)=-1.7V). The films were then reoxidized by immersion into an
acetonitrile solution containing 0.1 M TBABF.sub.4 and 0.001 M
diethylviologen diodide (E (2+/+)=-0.4 V). Infrared analysis showed
quantitative conversion of both polyisoimides to the poly-n-imide form.
EXAMPLE 3
Demonstration of the Catalytic Nature of Polysioimide Isomerization
An acetonitrile solution containing 0.1 M TBABF.sub.4 and 0.02 M
isophenylphthalimide was prepared. Under inert atmosphere the solution was
electrochemically reduced such that 3% of the isophenylphthalimide
molecules underwent reduction (E=-1.45 V). Infrared analysis of the
solution showed quantitative conversion to the n-phenylphthalimide form.
Thus, one electron transfer event can initiate at least 30 or more
isomerizations.
EXAMPLE 4
Electrodeposition of a Polyisoimide from THF
A solution containing 19% by weight of pure Thermid 6015 polyisoimide and
0.1 M TBABF.sub.4 was prepared in tetrahydrofuran (THF). The solution was
deaerated by N.sub.2 purge. Thermid 6015 has reductions at -1.0, -1.3, and
-1.8 V vs SCE. An optically transparent electrode was polarized to -1.6 V
to reduce the polyisoimide. Reduction was carried out for 10 min without
stirring to isomerize the polyisoimide to the poly-n-imide. This was
followed by polarization at 0 V t reoxidize any reduced poly-n-imide. The
electrode was removed and rinsed in THF for 2 min. and dried in vacuo at
80.degree. C. for 30 min. The film thickness was measured by profilometry
to be 25 .mu.m.
EXAMPLE 5
Electrodeposition of a Polyisoimide from NMP
A solution containing 3% Thermid 6015 and 0.15 M TBABF.sub.4 was prepared
in N-methylpyrrolidone (NMP). In this case, the Thermid was already
partially imidized (.about.40%) The solution was decerated by N.sub.2
purge. An optically transparent electrode was polarized to -1.3 V for
between 0 and 10 minutes to reduce the polyisoimide, followed by
reoxidiation at 0 V. The electrode was removed and rinsed in NMP and
acetone and dried in vacuo at 80.degree. C. for 30 min. The polyimide
thickness versus reduction time is shown in FIG. 1.
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